High-power lamps, to which the present invention relates, are to be considered lamps which operate in a power range of, for example, roughly 1000-4000 W, with a wall loading of 30-60 W/cm.sup.2. Lamps of this type are frequently used for intense illumination systems, e.g. for theatrical stages, to record scenes on film and for television, and for search lights or projection purposes. These lamps are usually coupled to optical systems, such as reflectors and lenses.
U.S. Pat. No. 4,686,419, Block et al, assigned to the assignee of the present application and the disclosure of which is hereby incorporated by reference, describes a high-pressure discharge lamp with a metal halide fill suitable for association with an optical system. These lamps, usually, have only a single bulb, that is, they are not covered by an outer bulb or cover element, in order to avoid, or at least minimize distortions arising in the optical system. Further, the electrode spacing of the discharge electrodes is as short as possible, for example in the order of about 3 cm. The discharge vessel is made of quartz glass, from which elongated and comparatively long cylindrical electrode shafts extend. Rather long molybdenum foils are melt-connected into the electrode shafts or extension. The lamps are complex to make and not subject to mass production; they are, each, made manually. When the lamp is operating, the temperature at the ends of the connecting foils which are r closest to the bases of the lamp must be below 400.degree. C. Due to the lack of an outer bulb, these ends are exposed to the oxygen in the air which tends to oxidize the lamp components, and thus limit the lifetime of the entire lamp. The melt-in technology for the long foils is complex, and thus the lamps become very expensive. Additionally, the lamps have a low lifetime, of about only 250 hours.
An additional disadvantage of these lamps occurs in operation, namely the relatively high electrical resistance of the long molybdenum foils results in high electrical losses in high power lamps. At 400.degree. C., these foils may have a resistance of about 0.043 ohms. The resulting electrical losses lead to heating of the lamp bulb extensions, which form the connecting shafts, and further contribute to reduction of the light output of the lamps. The light output of a typical lamp is in the order of about 80 1 m/W.
The unsatisfactory efficiency and the large dimensions of the lamps can be accepted for specialty applications, where their otherwise excellent characteristics outweigh the disadvantages. For other applications, however, particularly for outside illumination, where the lamps are exposed to wind loading, for example, their use was, heretofore, not justified.
A similar lamp, which also had a lifetime of only about 250 hours, but of even higher power, in the order of 4-12 kW, is described in U.S. Pat. No. 4,647,814, Dobrusskin et al; this lamp is described in detail, further, in the referenced "Technical-Scientific Publications of the OSRAM Company", which is a related company of the assignee of the present application. These publications are commercially available from the Springer Publishing Company.
It has previously been proposed, see German Patent Disclosure Document DE-OS 26 19 505, to limit the temperature of the lamps in the region of the bases to about 350.degree. C. by placing a plurality of gas-filled hollow spaces between the melt connection of the electrode and the base itself. Another arrangement is shown in German Patent Disclosure Document DE-OS 33 19 021 to reduce the temperature of the lamp extension or lamp shaft by forming the end surface of the electrode melt-in not as a flat and mirror surface but, rather, in funnel or conical shape. The melt-in extension in this lamp is a solid cylinder. Forming the end surface conically avoids back reflection from the previously known flat surface, which somewhat reduces the temperature loading of the lamp connecting extension. A full cylindrical lamp shaft acts like a light guide into which heat and light from the discharge volume is transmitted and coupled, resulting in heat transmission problems by the light shaft itself. In spite of the conical end surface, a 2500 W lamp still requires lamp shafts of about 11 cm length.
A metal halide high-pressure discharge lamp suitable for general illumination is described in European Published Application EP-OS 159 620. This lamp has high efficiency and includes an outer envelope or a second outer bulb. Placing a second outer bulb about the lamp substantially reduces the problem of oxidation due to oxygen in the air and permits a lifetime of several thousand hours; such a lamp is not, however, suitable for association with optical systems since the outer envelope or bulb substantially degrades the optical quality thereof. The bulb extensions or bulb shafts holding the electrodes can be reduced in length and they can be made in pinch or press technology, which can be carried out readily by machinery, and hence are relatively inexpensive. Yet, the temperature at the end of a pinch seal is substantially higher than 350.degree. C. This does not matter in a double-bulb lamp due to the atmosphere between the discharge vessel or discharge bulb and the outer envelope or surrounding bulb, which atmosphere may be inert or, effectively, may be absent, that is, the space between the discharge vessel and the outer bulb may be evacuated. The electrode spacing is substantial, in the order of about 10 cm. The lamp operates with high supply voltages, of about 380 V, and provides light output similar to the previously described single bulb lamps, namely about 85 1 m/W of the overall system. The lamp cannot be used effectively for optical applications where the lamp must cooperate with optical systems, such as a reflector, curved mirror or the like, due to the dual-envelope or bulb structure and the long arc length. The short overall construction length of the lamp results, however, in low wind loading so that this lamp is suitable for floodlights, outside illumination of buildings, monuments and the like.